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Posted: Dec 19, 2014

Shapeshifting metal nanoparticles eating tracks in graphene

(Nanowerk Spotlight) A widely discussed method for the patterning of graphene is the channelling of graphene by metal nanoparticles in oxidizing or reducing environments. In a previous Nanowerk Spotlight we reported on work by a group of researchers in Denmark who managed to observe this channelling process by silver nanoparticles in real time ("Nano-PacMan caught on video – observing nanoparticle channelling in graphene in real-time").
Catalytic channeling by oxidation or reduction is currently the only technique that produces channels with crystallographically oriented graphene edges with a roughness of less than 1 nanometer and a uniform width in the nanometer scale determined by the diameter of the nanoparticle. For any possibility of control of this phenomenon, the atomistic processes need to be better understood.
In follow-up work earlier this year in The Journal of Physical Chemistry ("Graphene Edges Dictate the Morphology of Nanoparticles during Catalytic Channeling"), the team from the Technical University of Denmark (DTU) reported that nanoparticles during etching of graphene have several peculiar properties.
"When the particles move, they seem to be shaped like a tiny snow-plow, with a V-shaped front," Peter Bøggild, a professor at DTU Nanotech, tells Nanowerk. "During channeling, the facets align closely with the zig-zag edges of the graphene sheet. When the particles stop or turn, they assume a round shape."
"So, the V-shaped front is not really due to the crystal structure of the particle, but is determined by the graphene lattice, with the the silver particle always touching the zig-zag edges of the graphene," he continues. "We also observed the particles becoming more 'liquid-like' during a turn – i.e. when they go from one zig-zag direction to another; which implies a 60 or 120 degree turn due to the hexagonal symmetry of the graphene sheet."
As a follow-up to their 'PacMan' video mentioned above, the researchers have now produced a new video that takes a closer look at the shape of the silver nanoparticles as they 'eat' through graphene:
A recent paper in Nature Materials ("Liquid-like pseudoelasticity of sub-10-nm crystalline silver particles") by researchers from China, US and Japan helped to explain the mysterious behavior of these nanoparticles: that the particles can change shape as if they were liquid, while they are in fact crystalline. This happens even at room temperature.
In the experiments done at DTU the particles kept their crystallinity beyond 900 K, while still retaining the fluid-like behavior. Whenever they encounter amorphous carbon (as opposed to graphene) change from channeling along one crystalline direction to another, they lose the apparent faceted shape.
The Nature paper states that silver nanoparticles can be pseudoelastic: solid but capable of shape shifting, during stress, and recovering to the rest shape when the stress disappears. This explains why the silver nanoparticles in the DTU experiment – although they consist of tens of thousands of atoms – have their shape defined by the contact with a sheet of graphene; just a few hundred atoms actually touching each other at the carbon-silver interface.
"This is the secret behind the stable, directional channeling effect (the 'PacMan' effect) that allows the particles to keep going along the crystal direction," explains Bøggild.
If the particles had some fixed, faceted shape independent of the graphene lattice, the long, straight and perfectly crystallographically aligned tracks would be much more unlikely. This could lead to near-deterministic patterning of graphene with nanometer scale pattern dimension and angstrom-level roughness.
"The tendency of the graphene edge to strongly dominate the silver particle shape could be used in combination with monodisperse particle deposition in specific places to achieve large-area nanopatterning of graphene with a predictability approaching that of state-of-art serial nanopatterning techniques, such as in situ lithography with electrons or ions," says Tim Booth, an associate professor at DTU Nanotech who led this work. "Additionally, the in situ formation of nanoscopic graphene structures by metal nanoparticles is of interest for the emerging field of graphene plasmonics."
There still is quite a knowledge gap to be filled: The scientists still don't know why the nanoparticles turn – whether this require the presence of defects or could happen even in pristine graphene.
Once they learn how to provoke such turning they would be able to construct a nanopencil with atomic resolution. In principle, many such nanoparticles can be started at the same time. In the future, this would allow to have massively parallel graphene patterning with atomic resolution, perhaps with the possibility of changing the path (turning) millions of particles in unison – "although the fundamentally statistical nature of the patterning of materials atom-by-atom may pose some fundamental limitations," says Tim Booth.
Patterned graphene is relevant for particle filters, plasmonics, transistors, and ultrasensitive sensors. One perspective the team mentions is the possibility that this technique may work on other 2D materials, like hexagonal boron nitride and transition metal dichalcodenides.
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